This invention relates to a process for preparing an oxymethylene polymer having good thermal stability, comprising polymerization of trioxane in the presence of a thiocyanate (an ester of thiocyanic acid), and an oxymethylene polymer obtained by the process. In a preferable embodiment of the present invention, the thermal stability of the polymer can be further improved when an acetal, an oxathiolane, an oxathiane or an organic acid anhydride coexists with a thiocyanate, in the above polymerization. Irradiation with a light beam or radiation before and/or during the polymerization results in advantageous effect.
It has been well known that polyoxymethylene is obtained by polymerizing formaldehyde or trioxane (a cyclic trimer of formaldehyde). It has been also well known that a cationic catalyst such as general Lewis acid and its complex, or a radiation is applied in the above polymerization process. However, the polymer obtained in the above process has poor chemical and thermal stabilities, and is easily depolymerized to form formaldehyde. Therefore in order to make the above polymer suitable for practical use, further treatments to increase its stability must be applied. For instance, a technique is known, in which the ends of polyoxymethylene molecules are acetylated with acetic acid anhydride. On the other hand, another technique is also well known, in which a copolymer is obtained by adding a second component which is copolymerizable with formaldehyde or trioxane. However, no process has been known for the polymerization of formaldehyde or trioxane in the presence of ester of thiocyanic acid. The use of alkyl-acetal as a chain transfer agent is well known in the polymerization of trioxane in the presence of a cationic catalyst (V. Jaacks et al. Makromal. Chem. 83, 56, 1965; Japanese Pat. No. 21638/1966 and others), and furthermore, it is known that the polymerization of trioxane, in the presence of a cyclic acetal, results in a copolymer (S. Rosinger et al. T. Polym. Sci. A-1, 5, 183, 1967; Japanese Pat. No. 3836/1969 and others), or that trioxane is copolymerized with a cyclic sulfur compound (Japanese Pat. No. 7631/1967).
However, the polymerization mentioned above has disadvantages -- that is, unpractical polymerization conditions, use of very reactive catalyst and the like. Although it is known that an aliphatic carboxylic acid anhydride can coexist in radiation polymerization of trioxane (Japanese Pat. No. 26594/1963), the acid anhydride, in this case, only acts to increase the polymerizable ability of trioxane.